Stroke is the number one cause of disability among Americans each year. Currently there is no therapy to cure stroke patients except the thrombolytic treatments, which have limited use. Our long-term goal is to promote functional recovery from stroke using human neural progenitor cells (hNPCs) as a potential therapy. We and others have shown that neural stem/progenitor in some cases can improve neurological function in rodents. However, transplant viability and functional outcome vary widely across studies. Our overall hypothesis is that hNPCs facilitate long-term functional by enhancing endogenous repair mechanisms through secretion of trophic factors. Including a focus on the trophic factors gives a mechanistic understanding of how transplanted stems cells augment endogenous repair processes. Importantly, we do not believe that the cells enhance recovery integrating into the host brain circuitry. In Specific Aim 1, we determine the effect of the transplanted cells on several endogenous repair mechanisms as well as the trophic factors expressed by the hNPCs in vivo over time, and then correlate these phenomena with functional recovery. We then test specific factors by manipulating their expression levels in hNPCs before transplantation. In Specific Aim 2, we determine the host microenvironment that is most conducive to cell-induced repair by varying the timing of transplantation post- stroke, with the goal of finding the optimal time to transplant. We also test the interplay between the host microenvironment and hNPCs by surveying host factors that are affected by hNPCs and also modifying the hNPCs'sensitivity to signals for migration and survival from the host's microenvironment. Together these aims will help identify the optimal time to transplant human neural progenitor cells after stroke and link successful cell therapy with critical molecular and cellular mechanisms that underlie endogenous repair after stroke. Graft survival and biology, and its effect on host repair mechanisms, will be assessed using immunohistochemistry. Functional recovery will be examined using behavioral tests. Our expertise in stroke research and cellular therapies (Kelly, 2004), neural stem cell biology and culture methods (Palmer, 2001), synaptogenesis (Christopherson, 2005), imaging (Micheva, 2007) and genetic manipulation of hNPCs (Suzuki, 2007) provide an excellent opportunity to develop a cross-disciplinary effort to study cell transplants for brain injury at Stanford.